Atomic beam resonator having a confocal conics field geometry in the second state selector

ABSTRACT

An atomic beam resonator having a source for producing the beam, a first and second state selector separated by an interaction zone, means for generating an oscillating field in the interaction zone, and a detector. The second state selector is a dipole magnet having pole pieces of trapezoidal shaped in such a way that, in a plane perpendicular to the beam axis, the field is described by a set of confocal conics, the equipotentials being hyperbolae and the field lines ellipses.

United States Patentv n91 Kartaschoff I I 11] 3,824,394 [451 July16,1974

1 1 ATOMIC BEAM RESONATOR HAVING A CONFOCAL CONICS FIELD GEOMETRY IN THESECOND STATE SELECTOR [76] Inventor: Peter Kartaschoff, Poudrieres 43,

Neuchatel, Switzerland 22 Filed: Jan. 30, 1973 21 Appl. No.: 328,087

[30] Foreign Application Priority Data Feb. 15, 1972 Switzerland 2110/72[52] US. Cl. 250/251 [51] Int. Cl GOln 27/78, l-lOls 1/00 [58] Field ofSearch 250/251; 331/3, 94

[56] References Cited UNITED STATES PATENTS Vanier 250/251 3,591,8007/1971 Kartaschoff et a1. 250/251 Primary Examiner-Wil1iam F. LindquistAttorney, Agent, or FirmBurns, Doane, Swecker & Mathis [57] ABSTRACT Anatomic beam resonator having a source for producing the beam, a firstand second state selector separated by an interaction zone, means forgenerating an oscillating field in the interaction zone, and a detector.

v The second state selector is a dipole magnet having pole pieces oftrapezoidal shaped in such a way that, in a plane perpendicular to thebeam axis; the field is described by a set of confocal conics, theequipotentials being hyperbolae and the field lines ellipses.

4 Claims, 5 Drawing Figures ATOMIC BEAM RESONATOR HAVING A CONFOCALCONICS FIELD GEOMETRY IN THE SECOND STATE SELECTOR The purpose of thisinvention is to obtain an improved efficiency together with a simpledesign of the second state selector of an atomic beam resonator, inparticular a cesium beam tube.

Cesium beam tubes are known as being absolute frequency standards ofhigh accuracy and are used as frequency determining elements in atomicfrequency and time standards or clocks.

The principle of operation of atomic beam resonators is well known andshall not be described here in detail. US. Pat. No. 3,591,800 containssuch a description and also discusses means to obtain an improvedefficiency of the state selectors. Referring to this earlier work, thepresent invention discloses an apparatus BRIEF DESCRIPTION OF THEDRAWINGS The accompanying drawings illustrate the prior art and variousembodiments of this invention. In such drawings: v

FIG. 1 is a cross-section of the second state selector described in theearlier patent (U.S. Pat. No. 3,591,800);

FIG. 2 shows a cross-section of a second state selector embodying thisinvention;

FIG. 3 shows a cross-section of another embodiment of this invention;

FIG. 4 shows a section along the beam axis of the state-selectoraccording to FIG. 3.

FIG. 5 shows a schematic view of an atomic beam resonator comprising thesecond state selector 16 embodying this invention, all other parts beingof prior art design.

The physical arrangement of the main components of an atomic beamresonator is shown schematically in FIG. 5. The atomic beam is formedbya source 13 which, in the case of a cesium beam resonator, consists ofan oven containing liquid cesium metal, heated to a temperatureproducing sufficient vapor pressure and having an aperture withcollinating channels to form the beam. The atomic beam passes through afirst state selector 14 magnet which in this example may be a quadrupoleor hexapole magnet whose symmetry axis coincides with the axis of thebeam.

It is well known that this kind of magnet has the property of focusingatoms which are in a higher hyperfine energy level by deflecting theseatoms 22 towards the axis, whereas the atoms 23 of a lower hyperfineenergy level are deflected away from the axis and eliminated. The atoms22 remaining in the beam pass through an interaction zone where they areexposed to an oscillating magnetic microwave field produced by means ofan external generator 21. If the power' level and the frequency of themicrowave generator are correctly adjusted, the atoms undergotransitions to the lower hyperfine level. The rate of transition isextremely sensitive to the frequency of the microwave field and itattains a sharp maximum at the frequency corresponding to the hyperfineenergy level separation.

The purpose of the second state selector 16 is to collect those atoms 24which have made the transition to the lower energy level on to adetecting device 18 and to eliminate the atoms 25 which have remained inthe higher level. In the case of cesium atoms, the detecting device 18consists of a hot wire or ribbon which ionizes the incident atoms, thelatter being collected on an electrode and producing a currentproportional to the rate of transitions. A beam stop 17 prevents fastundeflected atoms from hitting the detector and causing an unwantedbackground current.

The second state selector 16, which in FIG. 5 is shown only veryschematically, is the subject of the present invention and is thereforediscussed in more detail in the following paragraphs.

The atomic beam apparatus is enclosed in an evacuated envelope 19. Theoutput current is fed into an automatic frequency control system 20which acts on the microwave generator 21 to control the frequency insuch a way as to keep this frequency at the value producing the maximumtransition rate and hence the maximum output current. By this means, thefrequency 1 of the generator 21 is stabilized by means of the atomicbeam reasonator and the whole system can be .used as an atomic frequencystandard or clock, all as described in my earlier US. Pat. No.3,591,800.

The second state selector described in the earlier patent (U.S. Pat. No.3,591,800) and shown in FIG. 1 herein comprises a field concentratingrod 1 which is placed in the center of the gap formed by the concavepole pieces 2 and 3. .The axis of the beam is perpendicular to the planeof the drawing. The atoms pass through the gaps between 2, l and 1, 3respectively. The force acting on the magnetic moment of the atomscauses a deflection of those atoms which are in the desired quantumstate (i.e., those having the transition to the lower energy level)towards the detector element 18 of FIG. 5 placed along the axis of thebeam and located at some distancebehind the central pole piece 1. Theforce acting on the atom is proportional to the product of its magneticmoment and the gradient of the magnetic field. Thus the direction of theforce does not depend on the direction of the field itself but on thedirection of its gradient. In our case, the magnetic moment of the.atoms in the desired state is positive and the force acts in directionof increasing field strength.

This invention discloses a different and new design of the second stateselector which provides a similar result and avoids the disadvantages ofthe prior art device shown in FIG. 1. These disadvantages are thefollowing:

The central pole piece 1 obstructs a considerable fraction of thebeani'cross section. Almost half of the beam intensity is lost.

The means of supporting and aligning the central pole piece 1 arecomplicated and thus relatively expensive.

scribed in recent publications (R. Hyatt et al. Per- I formance of NewlyDeveloped Cesium Beam Tubes 3 and Standards, Proceedings of the 25thAnnual Symposium on Frequency Control, 26 28 April, 1971, US. ArmyElectronics Command, Ft. Monmouth, New Jersey) and in US. Pat. No.3,675,149 (Cutter et al.). In'practice, these disadvantages havebeenaccepted because the obtained efficiency was still very good compared tothat of earlier designs.

The present invention provides a similar high efficiency but avoids thedrawbacks mentioned above.

FIG. 2 illustrates the principle of this invention. The pole pieces 4and 5, made of a ferromagnetic alloy having a high permeability at highfields (e.g. Iron Cobalt Vanadium alloys) constitute equipotentialsurfaces which define the field in the region of the gap. The length ofthis assembly, in the direction of the z axis perpendicular to'the planeof the drawing, is assumed to be sufficiently large compared to the gapwidth so that fringe effects can be neglected. The pole pieces 4 and 5comprise flat surfaces only which are easy to machine. They constitutean approximation to a pair of surfaces having hyperbolic cross section.The field in the gap is thus defined by sets ofconfocal conics. Thecross sections of the equipotential surfaces are hyperbolae and thefield lines are ellipses. The detector filament is placed in the y-zplane at some distance beyond the magnet shown in the figure. Outsidethe y-z plane the field gradient has a component which is directedtowards this plane. The atoms to be detected are deflected in thedirection of the gradient and therefore in the direction of the y-zplane containing the detector element. Atoms in other states having anegative magnetic moment are deflected in the opposite direction awayfrom the detector element and are not detected.

In order to avoid detection of weakly deflected fast moving atoms, abeam stop 17 (see FIG. 5) may be placed in frontof the magnet and in they-z plane. However, the width (in the .r-direction) of such'a stop canbe just slightly larger than that of the detector filament.

The atoms hitting the beam stop 17 are lost, but this loss is negligiblecompared to the loss due to the wide central pole piece shown in FIG. 1.

Another embodiment of this invention is shown i FIG. 3. Here, the planeof symmetry containing the x and z axes of FIG. 2 is replaced by a flatpole piece 6. According to the mirror effect known from potential fieldtheory, the field in the remaining gap is the same as in the upper halfof FIG. 2, the other pole piece 7' being identical to 4 and 5.

Outside of the x-z plane the gradient is not everywhere parallel to thisplane but may be directed towards the tip of the pole piece. This causessome defocusing in the y-direction and the loss of atoms hitting thepole-piece. The useful deflection angles being very small, this loss isnot very important. It can be made negligible by'the modification shownin FIG. 4 which represents a section along the beam axis, i.e. in they-z plane, of the state selector shown in FIG. 3. Here the surface 8 ofthe pole tip 9 is not parallel to the plane atoms hitting the pole pieceis reduced.

All pole pieces shown in the FIGS. 2, 3 and 4 comprise only flatsurfaces of simple geometry. They are easier to manufacture andtherefore less expensive than the curved pole pieces used in earlierdesigns. One could be tempted to see in these designs a return to theold types of state selectors designed by Stern and Gerlach and Rabi,which are described in N. F. Ramseys book Molecular Beams (Oxford 1956,pp 394-396). However, in this invention the field is well described bysets of confocal conics, and this is not the case in the tialcharacteristics thereof. The presently disclosed embodiments aretherefore to be considered in all respects as illustrative and notrestrictive, the scope of the invention being indicated by the appendedclaims rather than by the foregoing-description and all changes whichcome within the meaning and range of equivalency of the claims aretherefore intended to be embraced therein.

I claim:

1. In an atomic beam resonator comprising a source means for producingand projecting a beam of atoms over an elongated predetermined beampath, first state selector means adjacent to said source for deflectingatoms in said beam, a second state selector means downstream of saidfirst state selector means for deflecting atoms in said beam, said stateselector means being separated in order to provide an interaction zone,means for generating an oscillating magnetic field in said interactionzone, and a beam detector device downstream of said second stateselector means for detecting only atoms in said beam having undergonetran sitions between two magnetic hyperfine energy levels bly forproducing a magnetic field through which said beam path extends, atleast one of said magnetic pole pieces being trapezoidal in crosssection in a plane perpendicular to the axis of the beam path and havingflat external surfaces for producing a magnetic field described in saidplane by sets of confocal conics, the equipotentials of said field beinghyperbolic and the field lines elliptic.

2. An atomic beam resonator as set forth in claim I wherein said magnetpieces have flat surfaces approximating an equipotential surface ofhyperbolic cross section.

3. An atomic beam resonator as set forth in claim 1 wherein one of thepole pieces of said magnet assembly has flat surfaces approximating anequipotential surface of hyperbolic cross section and the other pole hasa plane surface.

surface of the flat pole piece 10. The gap width in- 4. An atomic beamresonator as set forth in claim 1 wherein the width of the gap betweensaid magnet pole pieces increases in the direction towards said detectodevice. a

1. In an atomic beam resonator comprising a source means for producingand projecting a beam of atoms over an elongated predetermined beampath, first state selector means adjacent to said source for deflectingatoms in said beam, a second state selector means downstream of saidfirst state selector means for deflecting atoms in said beam, said stateselector means being separated in order to provide an interaction zone,means for generating an oscillating magnetic field in said interactionzone, and a beam detector device downstream of said second stateselector means for detecting only atoms in said beam having undergonetransitions between two magnetic hyperfine energy levels by havinginteracted with said oscillating magnetic field, the improvement whereinsaid second state selector means comprises a two pole piece magneticassembly for producing a magnetic field through which said beam pathextends, at least one of said magnetic pole pieces being trapezoidal incross section in a plane perpendicular to the axis of the beam path andhaving flat external surfaces for producing a magnetic field describedin said plane by sets of confocal conics, the equipotentials of saidfield being hyperbolic and the field lines elliptic.
 2. An atomic beamresonator as set forth in claim 1 wherein said magnet pieces have flatsurfaces approximating an equipotential surface of hyperbolic crosssection.
 3. An atomic beam resonator as set forth in claim 1 wherein oneof the pole pieces of said magnet assembly has flat surfacesapproximating an equipotential surface of hyperbolic cross section andthe other pole has a plane surface.
 4. An atomic beam resonator as setforth in claim 1 wherein the width of the gap between said magnet polepieces increases in the direction towards said detector device.